Introduction to Software Engineering (2+1 SWS)Winter Term 2009 / 2010 Dr. Michael EichbergVertretungsprofessur Software EngineeringDepartment of Computer ScienceTechnische Universität Darmstadt

Dr. Michael EichbergFachgebiet Software EngineeringDepartment of Computer ScienceTechnische Universität Darmstadt

The Factory MethodDesign PatternFor details see Gamma et al. in “Design Patterns”

|The GoF Design Patterns

The Factory Method Design PatternExample / Motivation

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Let’s assume we want to develop a framework for applications that can present multiple documents to the user (MDI style).

We want to support a wide variety of applications:▶ Text editors ▶ Word processors▶ Vector drawing applications▶ Document Viewers▶ ...

Our framework should - in particular - be able to manage the documents.

|The GoF Design Patterns

The Factory Method Design PatternExample / Motivation - Common functionality for handling documents

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TextMate Nisus Writer Pro

|The GoF Design Patterns

The Factory Method Design PatternExample / Motivation - Common functionality for handling documents

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TextMateNisus Writer Pro

(In the following, we focus on the implementation of “New”.)

|The GoF Design Patterns

The Factory Method Design PatternExample / Motivation - A Possible Implementation of the Framework

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public abstract class Document { public abstract void open(); public abstract void close();}

public abstract class Application { private List<Document> docs = new ArrayList<Document>(); public void newDocument() { Document doc = createDocument(); // the framework manages the documents docs.add(doc); doc.open(); } ... public abstract Document createDocument(); // factory method}

|The GoF Design Patterns

The Factory Method Design PatternExample / Motivation - Implementation of an Application Using the Framework

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public class TextDocument extends Document { … // implementation of the abstract methods}

public class MyApplication extends Application {

public Document createDocument() { return new TextDocument(); }

}

|The GoF Design Patterns

The Factory Method Design PatternExample / Motivation - Class Diagram of an Application Using the Framework

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open()close()save()

Document

TextDocument

createDocument()newDocument()

Application

createDocument()

MyApplication

docs

|The GoF Design Patterns

The Factory Method Design PatternIntent

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Define an interface for creating an object, but let subclasses decide which class to instantiate. (Factory Method lets a class defer instantiation to subclasses.)

|The GoF Design Patterns

The Factory Method Design PatternStructure

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Product

ConcreteProduct

factoryMethod()anOperation()

Creator

factoryMethod() ConcreteCreator

«method»... factoryMethod()...

|The GoF Design Patterns

The Factory Method Design PatternParticipants

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▶ Product… defines the interface of objects the factory method creates.

▶ ConcreteProduct… implements the Product interface.

▶ Creator… declares the factory method, which returns an object of type Product. Creator may also define a default implementation of the factory method that returns a default ConcreteProduct object.

▶ ConcreteCreator… overrides the factory method to return an instance of a ConcreteProduct.

|The GoF Design Patterns

The Factory Method Design PatternConsequences (I)

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▶ The framework’s code only deals with the Product interface; therefore it can work with any user-defined ConcreteProduct class.

▶!Provides a hook for subclassesThe hook can be used for providing an extended version of an object.

|The GoF Design Patterns

The Factory Method Design PatternConsequences (II)

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▶ Connects parallel class hierarchiesClass Name

Collaborator ACollaborator B

Responsibility A

Responsibility B

Responsibility C

drag()...

Manipulator

LineManipulator

createManipulator()

Figure

TextManipulator

Client

createManipulator()

Text

createManipulator()

Line

|The GoF Design Patterns

The Factory Method Design PatternImplementation

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Two major variants:▶ Creator is abstract▶ Creator is concrete and provides a reasonable default

implementation

|The GoF Design Patterns

The Factory Method Design PatternImplementation - Parameterized factory methods

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(E.g. imagine an document previewer which can handle very different types of documents.)

General form:public abstract class Creator { public abstract Product createProduct(ProductId pid);}

Applied to our example:public abstract class Application { public abstract Document createDocument(Type e);}public class MyApplication extends Application { public Document createDocument(Type e){ switch(e) { case Type.JPEG : return new JPEGDocument(); case Type.PDF : return new PDFDocument(); } }}

|The GoF Design Patterns

The Factory Method Design PatternImplementation - Parameterized factory methods

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public abstract class Application {

private Class<? extends Document> clazz;

public Application(Class<? extends Document> clazz){ this.clazz = clazz; }

public abstract Document createDocument(){ return clazz.newInstance(); }}

It is possible to use Java reflection in a type safe way.

|Placeholder

The Factory Method Design PatternRelated Patterns

• Factory Methods are usually called within Template Methods.•Abstract Factory is often implemented with factory

methods.

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